1. Field of the Invention
This invention relates to metallized dielectric wound capacitors and more particularly to wound capacitors in which each dielectric web is metallized on both sides.
2. Background Art
In making wound capacitors and particularly pulse and AC wound capacitors, a critical problem has been in forming the lead termination. The ends of the wound capacitor have been sprayed with molten metal particles to form terminals engaging the electrodes metallized on the dielectric web. Leads have then been bonded to the terminals. In order to decrease the ESR (equivalent series resistance), decrease the dissipation factor and increase the reliability of the connection between the metallization and the spray, it has been crucial to have a substantial amount of metallization defined at the capacitor end, since it is such metallization which is in electrical connection with the metal spray. The art has sought a high quality connection with as low resistance as possible. This is particularly important with thin film dielectrics and low voltage capacitors requiring low losses and low ESR.
In order to assure at least one thickness of metallization at each capacitor end the dielectric webs have been offset one from the other. This has been particularly important in view of material distortion or irregularity and travel of one dielectric web with respect to the other as a result of irregularities in the winding process caused, for example, by machine wear. The dielectric webs have been offset so that each metallized edge extends outwardly. Accordingly, even if the winding machine causes a major amount of irregularity there would still be an exposed edge of metallization at the capacitor end. However, such offset is objectionable when making small sized wound capacitors, since it substantially decreases the volumetric efficiency of the capacitor. A conventional offset can increase the size of such capacitors by approximately 20%.
It has been known to provide wound capacitors with each dielectric web metallized on both sides. Capacitors having various forms of dual metallization are described in the following U.S. Patents:
U.S. Pat. Nos. 4,012,817, 4,037,298, 3,179,862, 2,226,589, 3,435,308, 4,064,606, 2,887,649, 2,919,390, 3,348,284, 3,317,983, 3,178,623, 3,857,074, 3,581,167, 3,851,363.
Wound capacitors having dual metallization substantially provide double the surface area in contact with the metal spray which increases the reliability of the connection and also reduces the ESR of the capacitor. However, such dual metallization has left much to be desired in view of undesirable metal shorts which form between the metallized webs by way of pinholes in the dielectric web. Specifically the pinholes fill with metal which formed a connection path or short between the two metallized electrodes which connection path must be cleared. Such a clearing process is accomplished by applying a high voltage to the electrodes across the pinhole so that the metal therein burns and vaporizes away. However, such high voltage clearing process has been unacceptably expensive and produces undesirable contamination and burning of the dielectric. Further, clearing may not even be possible if the voltage rating of the dielectric is not of sufficiently high values.
Accordingly, an object of the present invention is a wound capacitor in which each dielectric web is metallized on both sides and does not require clearing.
Another object of the invention is a wound capacitor having dielectric webs metallized on both sides which are not offset one from the other in the winding process yet permitting sufficient exposed metallization even if the webs wander during the winding process or there is film distortion.